1. Field of the Invention
This application relates to new methods and systems for forming process gases having high effective ozone content. More particularly, it concerns new methods for the production of gases containing absolute partial pressures of ozone of at least 0.2 bar and systems for performing such methods.
2. Description of the Prior Art
Ozone (O.sub.3) is the triatomic form of oxygen and an allotrope of oxygen. Ozone is a very strong oxidant and like some other oxidants, for example, hydrogen peroxide, is not very stable and self-decomposes back into diatmoic oxygen.
Temperature affects dramatically the self-decomposition of ozone. For example, at temperatures of 100 to 200.degree. C., the rate of self-decomposition is very fast, in the range of seconds, while at temperatures below 50.degree. C., the half life time is long (minutes). At 350.degree. C., the self-decomposition of ozone is instantaneous so industrial, strictly thermal, waste ozone destructors operate at or above this temperature. Pressures higher than ambient pressure also accelerate the self-decomposition of ozone.
Large quantities of ozone are industrially produced in so called ozone generators, where an oxygen containing feed gas is subjected to electrical discharge, which transforms some of the contained oxygen into ozone. Normal feed gases to ozone generators are dried air, high purity oxygen, and mixtures of oxygen with nitrogen, carbon dioxide, argon, and other minor components.
Ozone concentrations that can economically be achieved with standard ozone generators range from 1 to 2% by weight in air and 2 to 4% by weight in high purity oxygen. So called, high concentration ozone generators produce ozone at concentrations of 6% by weight or higher. High concentration ozone generators must operate at higher power densities to achieve ozone concentrations of 6 or more % by weight than standard ozone generators. However, higher power densities result in higher ozone gas discharge temperatures and, therefore, high concentration ozone gas leaves the ozone generator at temperatures close to or above 50.degree. C. and ozone self-decomposition may take place.
Ozone generators must be cooled with a fluid coolant in order to eliminate the excess electrical power supplied to the ozone generator and which has not been transformed into chemical energy, ozone and other chemical species. Such coolant is generally water, chilled or not, but other liquids or gases may be used as coolants. The temperature and the flow of the coolant affect dramatically the ozone concentration which can be achieved and the temperature of the ozone gas leaving the ozone generator. Typical coolant temperatures range from 4.degree. C. for chilled water up to 30.degree. C. or higher. Heat created in operation of ozone generators has been removed using coolants in a variety of ways, e.g., see U.S. Pat. Nos. 1,994,462; 2,345,798; 2,404,773 and 2,704,274.
Ozone gas is economically produced at pressures ranging between about 1 and 3 bar absolute. Higher pressures may be non-economical and may result in undesired ozone self-decomposition, but ozone generators operating at higher pressures have been disclosed, see U.S Pat. Nos. 2,936,279; 4,614,573 and 4,988,484.
Some industrial applications of ozone, for example, medium consistency pulp bleaching, require ozone to be supplied at absolute ozone partial pressures ranging from 0.2 bar up to 0.7 bar or even higher up to more than 1.0 bar. In an industrial high concentration ozone generator, e.g., see U.S. Pat. No. 4,650,648, ozone can, for example, be produced at 7%-12% by weight and 2 bar absolute with a corresponding absolute ozone partial pressure of approximately 0.09 bar. For this reason, it is necessary in this bleaching application and in others to increase the ozone gas pressure after the ozone generator from 2 bar absolute to pressures ranging from about 4 up to about 15 bar absolute.
The compression of gases containing ozone and oxygen to provide a process gas to be subsequently used for some purpose has long been known, see U.S. Pat. No. 430,387. The present invention uses compression of such gases in a unique manner efficiently and economically to produce process gases containing ozone at relatively high absolute partial pressures.